Cassette swap - 24" Specialized Hotrock

Just got my son a 2008 Specialized Hotrock 24" 7 speed. He had a 20" prior and it's gearing combo was just right for our hill climbs here in central NC. I've noticed that the 28t granny gear on his 24" takes a little more pedal power with 24" wheels. I'd like to add a 12-32, 7 speed cassette. I've seen cassette swaps on this bike but I'm not sure if it's a direct bolt up on to the wheel and if a different length chain is required.

The specs for the cassette I'd like to add: SRAM PG-730 7 Speed 12-32 Cassette

You'll need a freewheel (screws directly onto hub) not a cassette (slides onto the hub). You probably will need a longer chain. It is possible that your derailleur would not extend out are enough to accommodate a 32-tooth cog.

Thanks for the info so far. The 34t might do the trick. We just got back off a 10 mile single track ride at the White Water Center in Charlotte, NC. The climbs are short but steep and he went down once near the top of a climb. I'm sure he would have made it with some better suited gearing.

Thanks for the info so far. The 34t might do the trick. We just got back off a 10 mile single track ride at the White Water Center in Charlotte, NC. The climbs are short but steep and he went down once near the top of a climb. I'm sure he would have made it with some better suited gearing.

Just a fyi, you need a new rear derailleur and chain for that mega range.

I'll weight the difference if I can find the gram scale.(former weight weenie here) Even it doesn't shave any weight at all I still know for certain it will help him. He wants to do some racing this fall so I've upgraded his seatpost and bars and plan on a set of 24" Kenda Small Block 8 kevlar's and will keep it at that.

Not necessarily. Freewheels are heavy. The stock Hotrock 24 7 speed freewheel is 13-28T I believe. Going to a 7 speed 13-32T has larger steel cogs. It should come in over 500g. I believe the IRD 7 speed 13-24T is around 400g.

I'm having some reservations on the freewheel swap. I've been seeing some not so great reviews on the IRD's. I'm tempted now to get way off topic and upgrade the crankset instead. I have seen FMF crank arms with 110mm BCD, 33t chainring and will require a 68x113 ISIS bottom bracket. It would run about $60 for the complete set-up (not including chain if needed) It would be a 3 tooth drop so I wonder how much a difference it would make?

The FMF crank can't run a bash. I milled it a bit with a dremel so I could put a BBG bash on. It's not very pretty but it works fine.

If I had to do it again, I would use a Sinz, which can be run double sided.

For freewheels, I use the Shimano. Hunt around, it can be had cheap.

For your case, with a 36T chainring. Your son came from a 20" 36T chainring with 28T rear. The gear inches are about 2.0. To get a 2.0 gear inch with a 24" bike you need a 36T chainring with a 34T rear. You cannot achieve it with just a chainring change at 110BCD.

If your desire is to get the same effort level, then the freewheel is your best bet.

Tree fort bikes has the Mark 4 IRD. I can't say if it's better, but it's the newer version. I too have read the same problems with IRD freewheels, that is they stop working and spin freely. I saw someone said they "fixed" it by lubricating it, but others said that the spacer for the highest gear cracked, and there's really no fix for that.

Again, no experience with IRD, but the Shimano have held up very well.

Any of the newer Shimano ones will be fine. Just look for the capacity of it. The newer Shimano Tourney can handle 34T. You should be able to pick up an Alivio or something fairly cheap as well from older adult bikes.

I'm having some reservations on the freewheel swap. I've been seeing some not so great reviews on the IRD's. I'm tempted now to get way off topic and upgrade the crankset instead. I have seen FMF crank arms with 110mm BCD, 33t chainring and will require a 68x113 ISIS bottom bracket. It would run about $60 for the complete set-up (not including chain if needed) It would be a 3 tooth drop so I wonder how much a difference it would make?

OP, does the current chainset run 26/36/48 tooth rings? I hate that so many kids bikes come spec'd that way. Most adults do not ride 26/36/48 on trails. I don't understand why companies put such high gearing on a bike for children. Mtnbiker72's suggestion is worth a look. Get a set of 22/32/42 rings.

Here's quick video. I messed and used the memory card that can't write fast enough for the cam so it' freezes about 10th frame or so and makes us look a little slower but try and overlook it. You'll see where he could use more granny at the end of the video. It's so much steeper than the video appears!White Water Center 28t - YouTube

It depends what you want to use. It's hard to say, as all modern mountain bike derailleurs support 34T, even some older ones do. I have an old long cage derailleur from 1990 that supports 34T just fine. The docs for your derailleur says 28T is the max, but you can try it, move the arm and see how far it goes.

However, when I change the gears, I tend to use SRAM stuff. I have a SRAM X7 derailleur on my son's 20" bike right now. It works, but I wouldn't go crazy unless you want to.

I used these lasco cranks on my son's 24" bike. I replaced the big ring with a BBG. The small/middle rings are steel. With a sinz expert chromoly bottom bracket the set weighs about the same as a shimano slx crankset, just under 900 grams. Not as light as sinz cranks, but more versatile with the granny ring (necessary for our trails). 152mm is a good fit for a bike this size.

I've got the 14-34t installed along with a new chain. I had an older XT rear derailleur laying as the stock Tourney was too compact and could not handle the 34t jump. I had concerns prior to installation that the jump from 24t (gear 2) to the 34t (gear 1) was not going to very smooth but I was wrong. It's changing just as quick and smooth as the rest. Now they are calling for rain here next couple days...dang it!